1. Field of the Invention
This invention relates to device processing and, more particularly, to device processing requiring the production of highly resolved features.
2. Art Background
The fabrication of devices such as semiconductor devices and magnetic bubble devices rely, to a large extent, on lithographic processes. In these lithographic processes a resist material is irradiated with exposing energy, for example, X-radiation, electrons, or light, in a desired pattern. This exposure induces a chemical reaction in the irradiated area of the resist material. The chemical reaction allows development, i.e., delineation of the pattern by changing the solubility of the exposed area relative to the unexposed area and thus allows removal of the exposed area for a positive resist or the unexposed area for a negative resist. Numerous resist materials have been developed. For example, polymethylmethacrylate (PMMA) has been used as a resist material and has yielded relatively good resolution, i.e., resolution as measured by line width in the range 10,000 A to 1000 A on thick substrates (thicker than 10.mu.). Although PMMA has relatively high resolution, its sensitivity is somewhat low for applications which require high throughput.
PMMA has been modified in a variety of ways to produce a concomitant modification of properties such as sensitivity. For example, the basic monomer of PMMA, i.e., methylmethacrylate has been copolymerized with methacrylic acid. Although this copolymer does not display the excellent resolution of PMMA, it nevertheless exhibits an increased sensitivity and thus has been suggested for applications such as delineating connection lines for LSI (large scale integrated circuits) where very high resolution is not essential and where relatively high sensitivity is more desirable.
The lower sensitivity of PMMA has also been exploited in resist configurations designed to produce high resolution. For example, the use of a bi-level resist composition which includes an upper level of the copolymer of methacrylic acid and methylmethacrylate and a lower level of PMMA has been discussed. (See Grobman et al., Proceedings of IEEE 1978 IEDM, page 58.) When this resist composition is irradiated with a dose sufficient to expose the upper polymeric layer, relatively little chemical change is induced in the lower PMMA layer. (This occurs due to the lower sensitivity of PMMA.) The upper layer is developed and after this development step a solvent is introduced to dissolve the uncovered PMMA.
Since progressively better resolution affords the possibility of both very large scale integration and discrete devices that depend on very fine features, research has been vigorous both in bi-level schemes to enhance resolution and in investigation of resist compositions which alone afford increased resolution. Generally, resolutions in the range 2500 A to 20,000 A for bi-level schemes and 1000 A to 30,000 A for single resist compositions are presently achievable on substrates thicker than 10 .mu.m.